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772                            Appendix L

of compression. Such an engine uses the same substance over
and over again, and the cycle is strictly reversible, for we have
complete expansion to <r, and complete compression to a. Its






efficiency therefore is measured like the cycle of Carnot.
substance is water at a, and steam and water at other times.


P. 623. Ratio of Expansion in a Single Cylinder.—
As a result of many recent practical experiments, it is found that
a ratio of expansion of between 6 and 9 is the highest limit
which can be adopted with economy, though a ratio of 4 will
yield practically as good results.

P. 624. Theoretical Diagram.-—In order to make the cal-
culations from the preliminary diagram agree with practical
diagrams, Prof. Unwin introduces a fractional coefficient in the
formulge, which he calls the diagram factor, and which is deduced
from experimental results of engines similar to that under con-
sideration. The value of the factor varies from '85 to '95 in
good engines.

P. 637. Exhaust Lap.—In the case of a D valve, an early
cut-off to steam will catfse an early cut-off to exhaust (see Zeuner's
diagram, p. 660). In such a case, if a later compression point be
advisable, it may be necessary njpt only to eliminate exhaust lap
entirely, but actually to give a small opening to exhaust when the
valve is at mid stroke. Such opening is then termed negative lap,
and would be shewn on the steam circle in Fig. 653.